This invention relates to pneumatic tires and particularly to the processing of the tire to assure dynamic uniformity of the tire. More particularly, the invention relates to an apparatus and method for reducing radial force variations of the tire and a tire produced therefrom.
In pneumatic tires, components such as beads, inner liner, tread and sheets or plies of rubberized cords utilized in the carcass and belts are segmentally assembled. Such assemblage can result in a structural nonuniformities in pneumatic tires which can cause variations in force which a rolling tire generates in a direction perpendicular to the surface, such as a roadway, which the tire rolls on. Such variations are generally referred to as radial force variations. Radial force variations are anomalies in a tire which result from "hard" and/or "soft" spots in the tire due to structural nonuniformities such as inconsistent wall thickness, ply turn-up variations, bead set, ply arrangement and other deviations. When the radial force variations exceed an acceptable level, the ride of the vehicle can be adversely affected.
Excessive radial force variations are generally corrected on tire uniformity correction apparatus, described, for example, in U.S. Pat. Nos. 3,574,973, 3,725,163 and 4,458,451, where the radial force variations are detected, measured and corrected by removing selected portions of tread rubber by a pair of grinders, one located at each shoulder of the tire which effects a reduction in radial force variations to an acceptable level.
In typical tire uniformity correction apparatus, a tire is mounted on a rotatable axle and the tire is inflated and rotated against a loadwheel wherein the tire is loaded to a predetermined load against the loadwheel during about 2 revolutions and thereafter rotated under full load for 4 revolutions. These 6 revolutions are generally referred to as "warmup" time and is performed to relieve any "set" in the tire that may have occurred during storage. Detection for excessive radial force variations is started after the warmup is completed.
Radial force variations are transmitted to the loadwheel where the radial force variations are sensed by transducers, such as load cells, wherefrom electrical signals representing the magnitude of the measured radial force variations are generated and sent to a computer. The measurement of radial force variations is generally performed in from one to three revolutions of the tire on the spindle in the above described tire uniformity apparatus depending on the electrical circuitry design of the apparatus. The signals are sorted and compared to predetermined lower and upper limits of correctable radial force variations wherein the computer makes a grind or no-grind decision by comparing the actual measured radial force variations to the lower and upper limits. If the measured radial force variations do not exceed the lower limit, no grinding is performed. If the measured radial force variations exceed the upper limit, the radial force variations are considered noncorrectable and no grinding is done and the tire is also removed from the apparatus.
Grind instructions are generated when the measured radial force variations exceed the lower limit and are less than the upper limit. A pair of hydraulic cylinders are actuated wherein each hydraulic cylinder moves a corresponding rotary grinder to the shoulder of the tire. The grinders remove rubber from selected areas of the shoulders to reduce the radial force variations to an acceptable magnitude such as at or below the lower limit.
The time required to grind a tire is dependent on the amount of rubber to be removed from the tire to reduce the undesirable radial force variations to an acceptable level and the rotational speed of the tire which is typically set at a constant speed of about 60 rpm. The amount of rubber to be removed in a given time, generally termed the rubber removal rate, is dependent on the type of treads on the tires being processed (e.g., rib type, large block, or small block). The grinding on a small or large block tread type tire is limited by what is called the heel and toe effect wherein the grinding wheel pushes the rubber ahead of it resulting in more rubber being removed from the leading edge of the small block than is removed from the trailing edge. The rubber removal rate for a small block tread type tire has been found to be limited to about 1/2 pound of radial force variation per revolution at 60 rpm. It has been found that for a large block tread type tire, about 1 pound of radial force variation per revolution at 60 rpm can be removed and a rib tread type tire can allow for about 2 pounds of radial force variation removal per revolution at 60 rpm.
Selected portions of tread rubber are removed when a rotating grinding wheel contacts and removes rubber from the surface of the tread over a predetermined angle which is generally less than 180.degree. of the tread surface. Each such occurrence of such rubber removal is generally called a pass. The number of grinding passes to reduce excess radial force variations from a tire generally are from 1 to about 45 passes. Attempts to improve productivity of grinding a tire by increasing the rotational speed of the tire results in more grinding force applied to the tire thereby reducing the amount of rubber that can be removed from the tire per pass wherein the number of grinding passes are increased.
Grinding affects the appearance of a tire tread when undesirable radial force variations are removed and a tire with unacceptable appearance is produced which may be reclassified to a blemish or scrap tire or may need further processing. The appearance of a tire tread can be evaluated in terms of texture which is determined by visual inspection of a tire by a person trained in such techniques. In particular, the person inspects a tire for grinding flaws on the ground surface such as ridges that extend perpendicular to the centerplane of the tire. If the tire does not meet the inspector's visual criteria, the tire generally requires further processing by grinding the tire over the ground surface with a finer grit stone at lower force removal rates to achieve the desired texture.
Such ridges found in the ground surface of a tread have been the subject of such papers as "Friction and Abrasion", Rubber Chemistry and Technology, 41, 209 (1968) by A. Schallamach, and the ridges have been described as wavy ridges as discussed in an article "Ridge Formation During The Abrasion Of Elastomers", Rubber Chemistry and Technology, 55, 1055 (1982) by A. K. Bhowmick. Such ridges characterize an undesirable effect of grinding a tire wherefrom a tire may appear to be a blemished or scrap tire.
Another grinding effect affecting the appearance of block tread type tire when undesirable radial force variations are removed occurs when the grinding removes excessive rubber from an initial portion of the block which the grinding stone contact, generally termed the leading edge of the block wherein the molded radius of the leading edge is transformed to a larger radius. This rounded leading edge is generally measured as the length of the excessive ground portion of a partially deformed block element measured from the edge of the block. Generally, the maximum allowable length is less than about 0.125 inch wherein if the rounded leading edge exceeds the maximum length, the tire generally requires reprocessing until the rounded leading edge is acceptable. When a rubber block tread type tire is cut to examine a leading edge, a rounded leading edge can be characterized as having a radius of curvature greater than the radius of curvature of the molded edge.
Another grinding effect is reversion which is evidenced by white areas of the ground portion as seen under a microscope. Reversion is generally defined as the rubber exhibiting soft tacky stage once the rubber has been cured to a tack free tough stage characteristic of fully vulcanized rubber. Once rubber has begun to revert, many of its desirable properties are lost. Reversion can occur because of excess heat applied to a grind patch by a conventional grinding hereinbefore described. It has been found that polishing the tire after grinding the tire can remove the visual affects of reversion.
It has been found that the continual grinding of a small block tread type tire for about one minute to remove about 30 pounds of radial force variation increases the probability of blemishing a tire due to appearance. The industry is continually seeking methods to improve the appearance of tires that require grinding.
The grinding of the tire occurs in a patch which is generally less than 50% of the circumferential length of the tire whereby actual grinding occurs for less than 50% of the actual time of processing the tire for tire uniformity. It is desirable to diminish the nongrinding time and to increase the amount of rubber that can be removed from the tire without adversely affecting the appearance of the tire.